Wireless networks are currently transitioning from an architecture referred to as Third Generation (3G) to Fourth Generation (4G) architecture, commonly known as LTE (Long Term Evolution). The 3G architecture offers packet data services with transmission speeds lower than LTE. For voice calls, a voice-centric call processing technology called 1xRTT (single carrier Radio Transmission Technology) is used which is often referred to as 2.5G.
The transition to LTE can be uneven and take time due to the fact that mobile devices (commonly called UEs for User Equipment) and LTE network equipment do not yet support commercial-grade VoLTE (voice over LTE) for voice service. Thus, even if a network provider chooses to upgrade its equipment to implement a LTE network for higher-speed packet data services, it must also be able to provide voice service to any UEs that enter its service coverage area. In addition, the network provider must continue to provide existing service to LTE devices that want to send or receive calls from 2.5G/3G devices or take advantage of other 3G services.
Since the transition from 3G to LTE may take years, there are a variety of efforts underway to make it as seamless as possible for end users of mobile devices, as well as cost effective for network providers. Some UEs are designed with two receivers, one for exchanging signals with a 2.5G/3G network and the other for exchanging signals with an LTE network. These UEs are referred to as Dual-Receiver (DRx) UEs. One downside for this solution, however, is potential negative impact on UE battery life caused by the simultaneous operation of two receivers in certain scenarios. Thus, the same or other UE manufacturers have also designed Single Receiver (SRx) UEs for potentially reduced cost of goods and improved UE battery life. However, these UEs are not able to interact with both 2.5G/3G and 4G networks at the same time.
On the network side, LTE network equipment such as MMEs (Mobility Management Entity) and eNBs (eNodeB) must be able to interact effectively with 2.5G/3G network equipment like MSCs (Mobile Switching Center) and SGWs (Serving Gateway). One technique for providing this interaction is commonly called Circuit Switched Fallback (CSFB). CSFB provides a mechanism for connecting calls between 1xRTT and LTE UEs via a combination of 2.5G/3G and LTE network equipment. A solution for SRx UEs commonly referred to as an ‘S102 Interface based’ is shown in FIG. 1. In this figure, a UE equipped with CSFB capabilities is shown at 102. It is connected to E-UTRAN (eNB) 104 which is further connected to MME 106 and Serving/PDN Gateway 108. The S102 interface is provided between MME 106 and 1xCS IWS (Inter-working Server) 110, which provides the connection to and interoperability with 1xRTT MSC 112. This solution uses interaction between an MME and an MSC using an IWS. Thus, it requires much higher upfront expenditures and has not been deployed by all major network providers.
The overlap and interaction of 2.5G/3G and LTE network equipment can cause performance problems with regard to unnecessary and duplicate processing which increases end-to-end voice call setup latency for the end user. In addition, current solutions for SRx UEs are expensive and not deployed by some major service providers. Therefore, what is needed is a procedure whereby both DRx and SRx UEs attached to LTE network can access the 1xRTT network with reduced call setup latency.